The invention relates to a method for internal exhaust-gas recirculation and to a correspondingly designed internal combustion engine.
Throughout the world, the demands imposed on the purity of the exhaust gases from internal combustion engines are subject to increasing statutory control, which is becoming ever more stringent. One possible way of satisfying these demands with regard to NOx emissions consists in internal exhaust-gas recirculation. This involves a certain quantity of exhaust gas being admixed with the fresh gas in a cylinder of the internal combustion engine prior to ignition. The NOx emission can be considerably reduced if an optimum mixing ratio is used.
To produce the mixture, DE 34 01 362 C2 has disclosed a method for internal exhaust-gas recirculation in an internal combustion engine with gas exchange valves periodically controlled by a camshaft in accordance with the four-stroke principle, in which the quantity of exhaust gas which remains in addition to the fresh gas after the gas exchange in the cylinder has ended can be adjusted by means of the opening and closing times of the gas exchange valves. If, for example, an exhaust is not closed after exhausting of exhaust gas from the cylinder has concluded, but rather is kept open for an adjustable period of time during the subsequent intake stroke, a quantity of exhaust gas, which depends on this period of time, can be taken in together with fresh gas. Consequently, the opening and closing times of the gas exchange valves simultaneously control the gas exchange and the internal exhaust-gas recirculation. Their implementation therefore constitutes a compromise between optimum gas exchange and optimum internal exhaust-gas recirculation.
WO 03/040540 has disclosed a method for internal exhaust-gas recirculation in which an intake valve is open during the gas exchange exhausting of exhaust gas from the cylinder into the exhaust duct, in order for part of the exhaust gas to be exhausted into the intake duct, so that both fresh gas and exhaust gas are taken in from the intake duct during the next gas exchange intake. In this case, the intake valve is actuated during the gas exchange exhausting independently of the periodic gas exchange intake, with the start of the primary stroke of the intake valve lying in the range between 180° and 210° and the end of this primary stroke lying in the range from approximately 270° to 310° crank angle after top dead center of the ignition. However, this only allows moderate exhaust-gas recirculation rates at full load.